In plants for the production of lubricating oils, solvent dewaxing is employed to remove wax from the lubricating oil stocks. In such processes either distillate or residual stocks of practically any viscosity in the raw or refined state from any crude source can be dewaxed. These remarks also apply to the production of deoiled waxes or wax free of oil.
In the basic process, to which this improvement is an adjunct only, to remove wax from the lubricating oil stock, a dilution solvent is introduced into a wax bearing oil stream. The solvent employed generally consists of a mixture of methyl ethyl ketone and an aromatic solvent (benzol, toluol or a mixture of the two). Dilution solvent dosages vary from a solvent-to-oil ratio of 1.0:1 to about 10.0:1 depending on the nature and viscosity of the charge stock. Other ketones, such as methyl isobutyl ketone and other aromatics may be used.
In the dewaxing process, which is well known in the art (Petroleum Refiner, Volume 15, Number 6, June 1936, pages 205-209, inclusive and September, 1974 Hydrocarbon Processing at page 188) such typically produces, as result thereof, three streams: (1) a dewaxed oil with solvent, (2) oil free wax with solvent and (3) slack wax with solvent. Currently, these products from the solvent dewaxing process have the solvent removed therefrom in steam strippers in order that the solvent may be recycled to the dewaxing process. The recovered solvent must then be separated from the water before the solvent can be recycled to the dewaxing process. In such procedure, the water content of the solvent is a critical design parameter. Also see Petroleum Processing Handbook, W. F. Bland and R. L. Davidson, McGraw-Hill 1967 Pages 3-92 through 3-94.
In typical operation of a solvent dewaxing process, the dilution solvent is introduced into a wax-bearing oil stream in controlled amounts at selected points in the chilling cycle so as to insure a wax crystal structure and liquid viscosity most suitable for filtration. The chilled charged mixture of solvent and wax-bearing oil stream flows through a filter feed tank typically to enclosed drum-type rotary vacuum filters. In these filters compartmentalized, cloth covered drums rotate, partially submerged in enclosed filter cases. A wax-free oil filtrate solution is drawn through the filter cloth to filtrate tanks in which the vacuum, which induces filtration, is maintained. The wax cake deposited upon the drum during filtration is washed on the filter continuously and automatically with cold solvent to produce a low oil content wax product.
The filtrate from the filtrate receiver is pumped through the heat exchangers to evaporators for recovery of the solvent heat for evaporating the solvent from the dewaxed oil solution.
In this prior art process, where at least two streams (relatively oil free wax plus solvent and relatively wax free oil plus solvent) are steam stripped to remove the solvent from the respective streams, a major problem exists with respect to separating the stripped solvent from the water resulting from the use of stripping steam. One manner of accomplishing this is to incorporate a solvent fractionator in the system. It is a difficult problem to maintain a proper and minimum water level in the solvent recycled into the dewaxing process. Water is at least to some extent soluble in the solvent and the solvent cannot be entirely separated from the water because the mixture will form an azeotropic mixture.